Switching amplifiers, also know as Class-D amplifiers as the name implies, have an output that is switched or pulse-width modulated (PWM) at a frequency much higher than the frequency of interest. For example, in audio applications, these amplifiers will switch at typical frequencies of 250 kHz, while the audio band is limited to 20 kHz. These amplifiers are analogous to switching regulators, and receive similar benefits and disadvantages of such devices when compared to their linear counterparts. The main advantage of these switching amplifiers is their efficiency and its derivatives, i.e., lower thermal dissipation, battery life, smaller power supplies, size, weight, etc. The main disadvantage is cost and complexity.
Present modulation schemes utilizing either half or full H-bridge output stage topologies which switch in a "binary" fashion. In such a switching method, there are two valid states for the bridge, neglecting dead time, which are shown at 10 in FIG. 1 and FIG. 2. Although simpler to implement, the disadvantage of this solution is that there is always a current 12 provided to the load due to the voltage differential always provided across nodes OUTP and OUTN, shown at 14, illustrated in FIG. 3. For operation near zero crossing, or no audio signal, the majority of the current used is wasted, and is a drop in efficiency. As shown in FIG. 3, an output squarewave with a 50% duty cycle will spend 50% of the time period decaying the current in the inductor, and 50% of the period to re-establish the current in the opposite direction, this resulting in a time averaged current of zero.
Furthermore, this will require that the load be inductive. Consider a pure resistive load. Switching the H-bridge in a "binary" fashion would place the power supply voltage across the load (plus parasitics of the switch). Unlike the current waveform 12 shown in FIG. 3, the resulting current would be a squarewave with a magnitude equal to the power supply divided by the resistance of the load. For example, an H-bridge using a 5V supply driving a 40.OMEGA. load would see a current of about 1 Amp, and this is with no signal. Although the electrical equivalent of a speaker is somewhere between purely resistive and purely inductive, this would still prevent filterless operation of Class-D amplifiers in audio applications as the main benefit of efficiency is lost. Today, the problem is solved by providing some current limiting device in series with the speaker, usually a post-filter comprised of inductors and thereby creating a current flow shown at 12 in FIG. 3. The typical circuit topology is shown at 16 in FIG. 4.